Aluminum nitride, gamma-aluminum oxynitride (ALON), and TiN are well known ceramics for applications such as electronic substrates, optical windows, and crucibles. ALON has been disclosed in U.S. Pat. No. 4,241,000 as an abrasive grain.
The mechanical properties of ceramic materials have been improved in recent years as a better understanding has been gained as to the effects of processing on the final ceramic microstructure. It is well known that low levels of porosity and a fine grain size are required for optimal mechanical performance of a ceramic. Microstructures possessing both of these characteristics are not readily obtained because as temperatures are increased to promote the elimination of pores during sintering, grain growth is also accelerated. One technique used to overcome this difficulty has been the combination of different crystalline components to form a composite material.
Ceramics composed of different combinations of AlN, polytypes of AlN, and ALON have been described in the literature, but the mechanical properties obtained have not been exceptional compared to other ceramic materials: T. Sakai, in "Sintering Theory and Practice", Materials Science Monographs, edited by D. Kolar, S. Pejovnik and M. M. Ristic, Vol. 14, p. 591-596, Elsevier Scientific Publishing Co., Amsterdam (1982); D. Turpin-Lannay, et al., in "Ceramic Powders" edited by P. Vincenzini, p. 891-897, Elsevier Scientific Publishing Co., Amsterdam (1983).
It is believed the background art does not teach the use of composites in the system AlN/ALON/TiN as abrasives. This is not surprising since AlN is a relatively soft ceramic with a hardness of only 1200 kg/mm.sup.2. By comparison, Al.sub.2 O.sub.3 and SiC, two commonly used abrasives, have hardness values of 2000 kg/mm.sup.2 and 2950 kg/mm.sup.2, respectively. Thus, ceramics containing AlN would not be expected to perform well as abrasives.
U.S. Pat. No. 4,320,203 describes composite cutting tools from a different system (Al.sub.2 O.sub.3 -ALON-TiN). In Example 1 the performance of these materials is compared to a composite of AlN, ALON, and TiN. However, the composition of this composite, its porosity, and its grain size are not described. The performance was considered poor and the patentee cautions against compositions containing AlN.
The background art in the AlN/ALON/TiN system is product oriented to relatively large shapes and forms, for example electronic substrates and crucibles, rather than small particulate products such as an abrasive grit. The methods of production employ ceramic powders which are pressed or otherwise shaped to the desired form, and then sintered or reaction-sintered to densify them. The manufacture of individual abrasive grits by these techniques is impractical due to the size and numbers required. Also, crushing of larger articles, such as billets, to form small particles is impractical because of the strength and toughness of these materials. Another problem with this process is the high cost of sinterable AlN and TiN powders. For example, commercial AlN powders typically cost $55-65/kg and cannot be sintered without extensive milling and size classification. Powders which are readily sinterable may cost as much as $325/kg.
Single phase ALON made by conventional powder processing is disclosed, for example, in U.S. Pat. Nos. 4,241,000 and 4,481,300. Single phase AlN made by conventional powder processing is disclosed, for example, in U.S. Pat. Nos. 4,435,513; 4,478,785; and 4,519,966.
It is known in the patent literature and technical publications to use sol-gel processes for the preparation of spherical, nuclear fuel particles of the carbides and nitrides of uranium and thorium. Typically hydrous sols of uranium oxide and thorium oxide are co-dispersed with carbon, formed into spheres, then gelled and reaction-sintered to form a carbide or nitride sphere. Examples of this teaching include: U.S. Pat. Nos. 3,171,715; 3,331,783; 3,860,691; and 3,904,736. The final products are typically less than 95% dense.
The preparation of abrasives comprising alumina and other metal oxides by a sol-gel process is disclosed in U.S. Pat. No. 4,314,827.
It is believed the use of sol-gel processes to prepare particles from mixed sols of alumina/carbon or alumina/titania/carbon followed by dehydration and reaction-sintering to form dense ceramics in the AlN/ALON/TiN system has not been disclosed in the background art.